Fuel cells used in vehicles and others use gaseous hydrogen. Since gaseous hydrogen is very large in volume, compressed hydrogen gas is used. The volume thereof is however still large for practical use, leading to space problems. Liquid hydrogen is smaller in volume than gaseous hydrogen. Holding liquid hydrogen is however difficult and not suited for practical use. Thus, use of hydrogen in the solid state has been being studied and developed to provide reduced volume and enhanced ease of handling. Hydrogen is used in the solid state by being stored in alloys. Such alloys are called hydrogen storage alloys. Hydrogen storage alloys repeat absorbing and releasing hydrogen.
Hydrogen storage alloys of this type are disclosed in patent document 1. Patent document 1 relates to hydrogen storage materials, the method of making the same, and a hydrogen generation device. The disclosed hydrogen storage materials comprise a bulk of a metal Mg (magnesium) with a surface crystalline region X having a short distance between nearest neighboring atoms compared with the other region of the Mg bulk, and have the ability such that 100 parts by weight of the hydrogen storage material can absorb and release 7 parts by weight or more of hydrogen at temperatures lower than or equal to 300° C.
The hydrogen storage materials disclosed in patent document 1 however require heat and pressure for hydrogenation reaction combining the Mg bulk with hydrogen to form MgH2. In other words, the disclosed hydrogen storage alloys need thermal energy and pressure energy to absorb hydrogen. It is inconvenient to supply such energies each time hydrogenation reaction is required.
It is known to catalyze hydrogenation reaction using a catalyst such as Pd (palladium) or Pt (platinum). However, even with the use of such catalyst, hydrogenation reaction requires thermal energy and pressure energy. The need for energy for hydrogenation reaction combining hydrogen storage alloys with hydrogen is considered as a drawback.
Further, Mg has a high ability to store hydrogen, which is an advantage, but its ability to cause hydrogen to diffuse in it in the solid state is low so that it takes time to absorb hydrogen. Mg's low ability to cause hydrogen to diffuse in it in the solid state, or in other words, hydrogen's slowly diffusing in the solid-state Mg means that even though hydrogen is absorbed in Mg in a surface region of a Mg alloy, the hydrogen absorbed is not passed on to Mg in an inner region thereof. Thus, only Mg in the surface region of the Mg alloy combines with hydrogen to form MgH2, which functions as a barrier film preventing a further amount of hydrogen from being absorbed. Such slow diffusion of hydrogen in the solid-state Mg is also considered as a drawback.